How about leaks from an 'INVISIBLE' hose ? water pumps the sensible easy answer , but how does coolant rest on top of something that's on the bottom of the engine (oil pan)? Are you able to clean it off and (from a cold start) observe it from both the top and underneath (which may require removal of a shield for proper view) check both sides of the engine block lower rad hose and heater hoses carefully look at all areas of radiator (if it's blowing back onto engine while driving).What the heck.....go ahead and replace the water pump , if it still has coolant on top of the oil pan you know one of three things (1) the engines upside down (2) water pump installed incorrectly or (3) something else is leaking. Don't forget to check the air in your tires!